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There is a "significant probability" of another sudden eruption at Tongariro in the next week and eruptions could continue for months, scientists says.

The eruption at Te Maari crater about 1.30pm yesterday sent a three or four kilometre plume into the air and lasted for about five minutes, GNS Science confirmed today.

Vulcanologists from throughout the country, including those not associated with GNS, met this morning to discuss the recent activity.

"Progressive pressure may build up over time and we think that's what's happening," GNS vulcanologist Nico Fournier said.

"At the moment we can't make any judgement calls but the overall opinion is if there is an eruption it is unlikely to be quite big...the bigger the eruption the more signals you should get beforehand and we haven't seen that."

Signals included earthquakes, gases emitted and the ground swelling slightly.

"We haven't had any evidence of any of those at the moment," Fournier said.

Mt Ruapehu was also showing signs of activity and there was still an increased likelihood of an eruption.

GNS took samples of Ruapehu's Crater Lake yesterday and hoped to have a better insight once results were in next week, Fournier said.

While New Zealand vulcanologists did not usually have to keep a close eye on two volcanoes experiencing heightened activity at the same time, they were not letting anything slip, he said.

"We need to make sure we don't miss anything at Ruapehu while Tongariro is putting on a show."

ERUPTION DISRUPTION

The Tongariro Alpine crossing would remain closed until the middle of next week, unless the Department of Conservation was advised otherwise by GNS, programme manager for communication relations, Kim Alexander-Turia, said.

The main huts on the mountain, including Oterere and Mangatopopo were also closed.

The Whakapapa track to Tama Lake remained open but visitors should inform DOC's Ruapehu office when they planned to be on the track, Alexander-Turia said.

Civil Defence has cancelled its national advisory regarding ash fall as no ash is coming from the volcano.

All roads in the area were open but police urged anyone driving nearby to be cautious and to pull over to the side of the road to view the mountain.

Some Air New Zealand flights cancelled this morning were expected to resume this afternoon, including flights in and out of Taupo and Gisborne.

CLOSE ENCOUNTER

Among the estimated 400 people walking the Tongariro Crossing yesterday was a party of 100 children, teachers and parents from Tamatea Intermediate, in Napier, who were near Ketetahi hot springs when the crater erupted.

"We were right up there next to it," teacher Lomi Schaumkel said. "It was just amazing".

"It was pretty scary from where we were and it looked absolutely spectacular, the ash that came out. It really did look like one of those atom bomb explosions, and it made a rumbling sound. Some panicked, some didn't. Everyone came down safely."

English tourists Peter and Angie Glanvill reckoned they were only about 200 metres from the eruption.

"We were coming down the track and turned the bend to see it erupting," Glanvill said. "It was so incredibly slow-moving, like everything was in slow motion. We stood there for a moment and watched it, with a lot of others on the track."

His wife said: "It was the icing on the cake for us, we've never been close to anything like this."

A Kiwi film crew were in the middle of interviewing a geologist and a vulcanologist on the mountain when the crater erupted behind them.

The three crew from web-video production company 90 Seconds TV were making a film to encourage Australians to travel to the Taupo area when "she blew her top", chief executive Tim Norton said.

"There was a big boom, which lasted quite a while. There was quite a lot of euphoria, lots of screaming and yelling. There wasn't a lot of fear."

The fear came later, when the smoke and ash began to pour into the sky.

"When that happened, when they felt the rumble deep in the ground, that was a little bit frightening."